In a typical cellular wireless system, an area is divided geographically into a number of cell sites, each defined by a radio frequency (RF) radiation pattern from a respective base transceiver station (base station) antenna. The base station antennas of the cells may then be coupled with other equipment, cooperatively functioning as a radio access network (RAN), which may provide connectivity with a transport network such as the public switched telephone network (PSTN) or the Internet.
When a mobile station (such as a cellular telephone, personal digital assistant, or appropriately equipped portable computer, for instance) is positioned in a cell, the mobile station and base station can communicate with each other in various channels over the RF air interface. Communications from the base station to a mobile station are considered to be in a “forward” direction, so the air interface channels used to carry such communications are referred to as the “forward link” channels. Conversely, communications from an mobile station to the base station are considered to be in a “reverse” direction, so the air interface channels used to carry such communications are referred to as “reverse link” channels.
Communications between a mobile station and a base station will typically suffer from varying levels of interference and signal degradation, due to factors such as (i) the number and power level of mobile stations concurrently communicating over the air interface, (ii) obstructions such as buildings or foliage, and (iii) the distance between the mobile station and the base station. In order to account for this, the power level of signals transmitted between the mobile station and base station can be dynamically adjusted.
To facilitate this on either the forward link or reverse link, a receiving device may monitor the quality of the signals that it receives from the other end and may direct the other end to adjust transmit power based on the measured quality. For instance, a receiving end may monitor the frame error rate (FER) in communications that it receives from the other end and compare the measured FER to a target FER value. If the measured FER is greater than the target FER, then the receiving end may direct the other end to increase transmit power, in an effort to reduce the FER. On the other hand, if the measured FER is less than the target FER, then the receiving end may direct the other end to decrease transmit power, in an effort to avoid unnecessarily strong transmissions that could interfere with other communications.
By way of example, a RAN and mobile station may engage in a two-part power control process for reverse link transmissions. In this process, the base station (e.g., a base station controller) maintains a “setpoint” value, e.g., Eb/No, which is a decibel measure of the mobile station signal energy to noise (spectral density). The setpoint represents how strong the mobile station signal must be for the base station to be able to successfully receive and decode bits of data transmitted by the mobile station. Given a particular noise level in the air interface, if the received mobile station signal level is not high enough, the base station might not be able to make out the bits of the signal compared to the background noise.
A base station may have an initial setpoint designated by the manufacturer of the base station for use with respect to all mobile stations that the base station serves. Further, the base station may continuously estimate the noise level in the air interface, based on various factors such as FER or signal strength measurements reported by mobile stations, for instance.
When a mobile station is going to engage in a call (e.g., a voice call or data session), the mobile station may select an initial reverse link power level (i.e., the power level at which it will initially transmit to the base station) based on its measurement of received signal power levels. In particular, if the mobile station receives a relatively high strength signal from the base station, the mobile station might logically conclude that it is relatively close to the base station, so it might be programmed to initially transmit to the base station at a relatively low power level. Conversely, if the mobile station receives a relatively low level signal, it might be programmed to initially transmit to the base station at a relatively high power level.
The first part of the reverse link power control process is known as “open loop power control.” In open loop power control, the base station measures the power level of the signal that it receives from the mobile station, which will have degraded from the time that it left the mobile station. Given this value and the estimate of noise in the air interface, the base station may then establish a measured value of Eb/No, which the base station will compare to the setpoint. If the measured Eb/No does not match the setpoint, the base station will instruct the mobile station to adjust its transmit power, typically by a predetermined increment. The base station will then establish a new measured Eb/No value and compare it to the setpoint, repeating the process until the measured Eb/No matches the setpoint.
For instance, if the measured Eb/No is too low compared to the setpoint, then the base station may conclude that the mobile station needs to increase its transmit power and may therefore send to the mobile station a signaling message (e.g., a control bit in a predetermined air interface timeslot) instructing the mobile station to increase its transmit power, by 1 dB or by some other defined increment. Similarly, if the measured Eb/No is too high compared to the setpoint, then the base station may conclude that the mobile station needs to decrease its transmit power and may therefore send to the mobile station a signaling message instructing the mobile station to decrease its transmit power, again by a 1 dB or another defined increment.
As an example, assume that (i) the initial setpoint is 5 dB, (ii) the mobile station transmits at 10 dBm and (iii) the estimated noise level is −65 dBm. Assume then that the base station measures received mobile station signal energy of −61 dBm. The base station may therefore compute a measured Eb/No of 4 dB, which is lower than the setpoint by 1 dB. Consequently, the base station would instruct the mobile station to raise its transmit power by 1 dB. As a result, the base station might then measure received mobile station signal energy of −60 dBm. And the base station may then compute a measured Eb/No of 5 dB, which matches the setpoint.
While seeking to match the setpoint, the base station and the mobile station may also engage in the other part of the power control process, which is known as “closed loop power control.” In closed loop power control, the base station continuously measures the FER of received mobile station signals, i.e., the percentage of frames on the reverse link that are in error, and the base station compares the measurement to a predetermined FER threshold known as a “target FER”. If the measured FER does not match the target FER, the base station will adjust the setpoint used in the open loop power control process, so as to cause the mobile station's transmit power to be adjusted in a manner helps equalize the FER. Thus, the base station will thereby use the target FER as a benchmark to trigger adjustment of the setpoint and thus to trigger an adjustment in mobile station transmit power.
In particular, if the base station determines that the measured FER is greater than the target FER, then the base station may conclude that the setpoint should be increased so that higher mobile station transmit power will be allowed and the FER can be decreased. Thus, the base station may increase the setpoint by an increment such as 1 dB for instance. As a result, the setpoint may be pushed higher than the measured Eb/No, and so, in the open loop power control process, the base station will instruct the mobile station to increase its transmit power, thus likely resulting in decreased FER.
On the other hand, if the measured FER is less than the target FER, then the base station may conclude that the setpoint can be reduced so as to limit the mobile station transmit power (and thus hopefully reduce noise that would be experienced by others) while maintaining transmission quality within the target FER. Therefore, in such a situation, the base station may reduce the setpoint, similarly by an increment such as 1 dB for instance. And as a result, the setpoint may be pushed lower than the measured Eb/No, so the base station will instruct the mobile station to reduce its transmit power, thus likely resulting in increased FER.